On 2024-09-29 17:51, Paul E. McKenney wrote:
On Sun, Sep 29, 2024 at 07:16:08AM -0400, Mathieu Desnoyers wrote:
Refer to ptr_eq() in the rcu_dereference() documentation.
ptr_eq() is a mechanism that preserves address dependencies when
comparing pointers, and should be favored when comparing a pointer
obtained from rcu_dereference() against another pointer.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: John Stultz <jstultz@xxxxxxxxxx>
Cc: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Zqiang <qiang.zhang1211@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: maged.michael@xxxxxxxxx
Cc: Mateusz Guzik <mjguzik@xxxxxxxxx>
Cc: Gary Guo <gary@xxxxxxxxxxx>
Cc: Jonas Oberhauser <jonas.oberhauser@xxxxxxxxxxxxxxx>
Cc: rcu@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: lkmm@xxxxxxxxxxxxxxx
Cc: Nikita Popov <github@xxxxxxxxxx>
Cc: llvm@xxxxxxxxxxxxxxx
---
Changes since v0:
- Include feedback from Alan Stern.
---
Documentation/RCU/rcu_dereference.rst | 32 ++++++++++++++++++++++-----
1 file changed, 27 insertions(+), 5 deletions(-)
diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst
index 2524dcdadde2..9ef97b7ca74d 100644
--- a/Documentation/RCU/rcu_dereference.rst
+++ b/Documentation/RCU/rcu_dereference.rst
@@ -104,11 +104,12 @@ readers working properly:
after such branches, but can speculate loads, which can again
result in misordering bugs.
-- Be very careful about comparing pointers obtained from
- rcu_dereference() against non-NULL values. As Linus Torvalds
- explained, if the two pointers are equal, the compiler could
- substitute the pointer you are comparing against for the pointer
- obtained from rcu_dereference(). For example::
+- Use operations that preserve address dependencies (such as
+ "ptr_eq()") to compare pointers obtained from rcu_dereference()
+ against non-NULL pointers. As Linus Torvalds explained, if the
+ two pointers are equal, the compiler could substitute the
+ pointer you are comparing against for the pointer obtained from
+ rcu_dereference(). For example::
p = rcu_dereference(gp);
if (p == &default_struct)
@@ -125,6 +126,23 @@ readers working properly:
On ARM and Power hardware, the load from "default_struct.a"
can now be speculated, such that it might happen before the
rcu_dereference(). This could result in bugs due to misordering.
+ Performing the comparison with "ptr_eq()" ensures the compiler
+ does not perform such transformation.
+
+ If the comparison is against another pointer, the compiler is
+ allowed to use either pointer for the following accesses, which
+ loses the address dependency and allows weakly-ordered
+ architectures such as ARM and PowerPC to speculate the
+ address-dependent load before rcu_dereference(). For example::
+
+ p1 = READ_ONCE(gp);
+ p2 = rcu_dereference(gp);
+ if (p1 == p2)
+ do_default(p2->a);
+
+ The compiler can use p1->a rather than p2->a, destroying the
+ address dependency. Performing the comparison with "ptr_eq()"
+ ensures the compiler preserves the address dependencies.
Bitter experience leads me to suggest a "// BUGGY" comment on the "if"
statement in the above example, and a corrected code snippet right here. :-/
Changing for the following:
+ p1 = READ_ONCE(gp);
+ p2 = rcu_dereference(gp);
+ if (p1 == p2) /* BUGGY!!! */
+ do_default(p2->a);
+
+ The compiler can use p1->a rather than p2->a, destroying the
+ address dependency. Performing the comparison with "ptr_eq()"
+ ensures the compiler preserves the address dependencies.
+ Corrected code::
+
+ p1 = READ_ONCE(gp);
+ p2 = rcu_dereference(gp);
+ if (ptr_eq(p1, p2))
+ do_default(p2->a);
Other than that, loks good!
Let me know if I should add an acked-by from you on this
documentation patch as well.
Thanks,
Mathieu
Thanx, Paul
However, comparisons are OK in the following cases:
@@ -204,6 +222,10 @@ readers working properly:
comparison will provide exactly the information that the
compiler needs to deduce the value of the pointer.
+ When in doubt, use operations that preserve address dependencies
+ (such as "ptr_eq()") to compare pointers obtained from
+ rcu_dereference() against non-NULL pointers.
+
- Disable any value-speculation optimizations that your compiler
might provide, especially if you are making use of feedback-based
optimizations that take data collected from prior runs. Such
--
2.39.2
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
